Most integrated receivers used on modern mobile user devices have to achieve high levels of sensitivity. The integrated receiver needs to distinguish between a particular band for receiving information of interest and the multitude of bands either transmitted on the same communication channel or in neighboring communication channels. Typically, a combination of Surface Acoustic Wave (SAW) filters and high-Q chip inductors have been provided to meet the required band sensitivity. If the integrated receiver has to receive information from various bands, the disadvantage to these solutions is that multiple receiver circuits are required for the multiple bands and/or communication channels.
Translational filters may also be utilized to provide the required sensitivity to the multiplicity of bands. These translational filters may provide a high-Q filter response by utilizing a mixer circuit to convert a low frequency impedance response into a desired frequency band. Translation filter operate by down converting an input signal associated with the RF signal in accordance with a first oscillation signal to intermediate frequencies (IF) or baseband frequencies and then filtering the down converted signal with a reactive circuit having an impedance response centered around the IF or baseband frequencies. The input impedance response presented to the input signal is an impedance image of the impedance response of the reactive circuit, which ideally is offset from the reactive circuit impedance response by the first oscillation frequency.
This arrangement is advantageous because often it is easier to filter at IF and baseband than at RF frequencies and thus the translation filter provides the advantages of low frequency filtering at RF frequencies. The reactive circuit of the translation filter may have reactive components with variable reactive impedance values thereby allowing the polyphase reactive circuit to vary the characteristics of the reactive circuit impedance response in accordance with the band or bands of interest. Also varying the first oscillation frequency allows the translational filter to center the characteristics of the input impedance response at different frequencies for different bands.
If the parasitic reactive impedances of the source are insignificant the arrangement works well and the first mixer circuit presents an input impedance response having a notch or a pass band(s) having a high-Q factor centered around the desired band. Unfortunately, this is not always the case, and the source may have reactive impedances that cause a discrepancy between the actual offset of the impedance image and the first oscillation frequency such that the desired band is blocked instead of received.
Another problem with the arrangement is that the impedance image may include negative frequency impedance response from the impedance response of the reactive circuit. Consequently, for example, if reactive circuit impedance response is a low pass impedance response that is translated by the mixer circuit into a bandpass impedance response centered at approximately the local oscillation frequency, the bandpass of the bandpass impedance response may have twice the bandwidth thus lowering the O-factor. Alternatively, if the reactive circuit impedance response is a bandpass impedance response, then the impedance image will include two bandpasses after being offset by the mixer, one from the positive frequency impedance response and another from the negative frequency impedance response of the reactive circuit impedance response. As a result, unwanted signals may be received due to the acceptance of the impedance image of the negative frequency impedance response.
What is needed is an RF circuit that permits the input impedance response to be further adjusted to compensate for the effects of the parasitic impedances of the source and/or also rejects the negative frequency impedance responses from the reactive circuit.